In the past, use has been made of pulse encoders combining the use of absolute type slits and increment type slits, as represented by the Gray code, to detect the rotational position or rotational speed of a servo motor for driving and controlling industrial robots, machine tools, etc.
In the past, however, to further raise the resolution of detection of the rotational position by the increment type slits in the fixed slit disk, the slits have been divided into a plurality of increment type slit groups, and these groups have been disposed around the optical axis of one light emitting diode so that the slit groups receive a substantially equal irradiation by the light emitting diode. Therefore, there has been no room for an irradiation of absolute type slits as well, and therefore, these absolute type slits had to be irradiated by using another light emitting diode.